So what exactly do extras talk about?

I'm sitting here watching an episode of Cheers on Nick-at-Night and I see all of these extras that appear to be engulfed in conversation. What exactly are they talking about? Do they stop talking when the director yells cut? Are they paid? Are all of their actions scripted (E.g. walking out the door)? Fascinating...

watemelon, peas and carrots, etc. Sometimes that's literally the case. Depends on the people you're working with. Some will self script a mini scene, for nobody but themselves, some are talking about where to get coffee after work.
All movement is blocked. Don't want to fuck up the directors composition. Do you stop talking when the director yells cut? Only if you want to keep your job.
T